A storage battery system including a power source switching device is known. The power source switching device selects one of output power from a commercial alternating current (AC) power source and output power from an auxiliary AC power source including a storage battery and an inverter, and supplies the selected one to a load. In a certain conventional method, in order to suppress the occurrence of arc discharge, the following processing is performed: in the case where power is supplied to a load via a relay disposed on a feeder wire, in order to stop the power supply to the load, the relay is disconnected after the current flowing on the feeder wire becomes zero (Patent Literature 1).